At base, a Molt command is a Rust function that performs some kind of work and optionally returns a value in the context of a specific Rust interpreter. There are two ways an application (or library crate) can define application-specific Rust commands:

• As a Rust CommandFunc function
• As a Molt procedure, or proc.

A CommandFunc command is any Rust function that implements CommandFunc:

# #![allow(unused_variables)]
#fn main() {
pub type CommandFunc = fn(&mut Interp, ContextID, &[Value]) -> MoltResult;
#}

For example, here's a simple command that takes one argument and returns it unchanged.

# #![allow(unused_variables)]
#fn main() {
fn cmd_ident(_interp: &mut Interp, _context_id: ContextID, argv: &[Value]) -> MoltResult {
    check_args(1, argv, 2, 2, "value")?;

    molt_ok!(argv[1].clone())
}
#}

The argv vector contains the arguments to the command, beginning with the command's name. The check_args method verifies that the command has the right number of arguments, and returns the standard Tcl error message if not. Finally, it uses molt_ok! to return its first argument.

Install this command into the interpreter using the Interp::add_command method:

# #![allow(unused_variables)]
#fn main() {
interp.add_command("ident", cmd_ident);
#}

A normal CommandFunc is useful when extending the Molt language itself; but application-specific commands need to manipulate the application and its data. In this case, add the required data to the interpreter's context cache. The cached data can be retrieved, used, and mutated by commands tagged with the relevant context ID.

The context cache is a hash map that allows the interpreter to keep arbitrary data and make it available to commands. The usual pattern is like this:

• The application defines a type containing the data the command (or commands) requires. We'll call it AppContext for the purpose of this example.

• The application saves an instance of AppContext into the context cache, retrieving a ContextID.

• The application includes the ContextID when adding the command to the interpreter.

• The command retrieves the AppContext as a mutable borrow.

// The AppContext
struct AppContext { text: String }

// The Command
fn cmd_whatsit(interp: &mut Interp, context_id: ContextID, argv: &[Value]) -> MoltResult {
    check_args(1, argv, 2, 2, "value")?;

    let ctx = interp.context::<AppContext>(context_id);

    // Append the first argument's string rep to the
    // AppContext struct's text field.
    ctx.text.push_str(argv[1].as_str());

    molt_ok!()
}

// Registering the command
fn main() {
    let interp = Interp::new();
    let id = interp.save_context(AppContext::new());

    interp.add_context_command("whatsit", cmd_whatsit, id);

    ...
}

The saved AppContext will be dropped automatically if the whatsit command is removed from the interpreter.

Any number of Molt commands can share a single cached context struct:

    let interp = Interp::new();
    let id = interp.save_context(AppContext::new());

    interp.add_context_command("first", cmd_first, id);
    interp.add_context_command("second", cmd_second, id);
    interp.add_context_command("third", cmd_third, id);
    ...

The context struct will persist in the cache until the final command is removed (or, of course, until the interpreter is dropped).

The standard way to represent an object in TCL is to define a command with attached context data. The command's methods are implemented as subcommands.

The context cache supports this pattern trivially. Define the object's instance variables as a context struct, and define a command to create instances.

// Instance Data
struct InstanceContext { text: String }

// Command to make an instance
fn cmd_make(interp: &mut Interp, _: ContextID, argv: &[Value]) -> MoltResult {
    check_args(1, argv, 2, 2, "name")?;

    let id = interp.save_context(InstanceContext::new());

    interp.add_context_command(argv[1].as_str(), cmd_instance, id);

    molt_ok!()
}

// Instance Command
fn cmd_instance(interp: &mut Interp, _: ContextID, argv: &[Value]) -> MoltResult {
    check_args(1, argv, 2, 0, "subcommand ?args...?")?;

    // Get the context
    let ctx = interp.context::<AppContext>(context_id);

    // Do stuff based on argv[1], the subcommand.
    ...
}

// Registering the command
fn main() {
    let interp = Interp::new();

    interp.add_command("make", cmd_make);

    ...
}

Then, in Molt code you can create an object called fred, use its methods, and then destroy it by renaming it to the empty string.

% make fred
% fred do_something 1 2 3
...
% rename fred ""

A Molt procedure is a routine coded in Tcl and defined using the proc command. A crate can compile Tcl procedures into itself using the include_str! macro. Start by defining a script that defines the required procedure, say, procs.tcl, and put it in the crate's src/ folder adjacent to the Rust file that will load it. The Rust file can then do this:

# #![allow(unused_variables)]
#fn main() {
let mut interp = Interp::new();

match interp.eval(include_str!("commands.tcl")) {
    Err(exception) => {
        if exception.is_error() {
            panic!("Couldn't load procs.tcl: {}", msg.value());
        }
    }
    _ => ()
}
#}